A major problem with colorants is that they tend to fade when exposed to electromagnetic radiation such as sunlight or artificial light and the like. It is believed that most of the fading of colorants when exposed to light is due to photodegradation mechanisms. These degradation mechanisms include oxidation or reduction of the colorants depending upon the environmental conditions in which the colorant is placed. Fading of a colorant also depends upon the substrate upon which they reside.
Product analysis of stable photoproducts and intermediates has revealed several important modes of photodecomposition. These include electron ejection from the colorant, reaction with ground-state or excited singlet state oxygen, cleavage of the central carbon-phenyl ring bonds to form amino substituted benzophenones, such as triphenylmethane dyes, reduction to form the colorless leuco dyes and electron or hydrogen atom abstraction to form radical intermediates.
Various factors such as temperature, humidity, gaseous reactants, including O.sub.2, O.sub.3, SO.sub.2, and NO.sub.2, and water soluble, nonvolatile photodegradation products have been shown to influence fading of colorants. The factors that effect colorant fading appear to exhibit a certain amount of interdependence. It is due to this complex behavior that observations for the fading of a particular colorant on a particular substrate cannot be applied to colorants and substrates in general.
Under conditions of constant temperature it has been observed that an increase in the relative humidity of the atmosphere increases the fading of a colorant for a variety of colorant-substrate systems (e.g., McLaren, K., J. Soc. Dyers Colour, 1956, 72, 527). For example, as the relative humidity of the atmosphere increases, a fiber may swell because the moisture content of the fiber increases. This aids diffusion of gaseous reactants through the substrate structure.
The ability of a light source to cause photochemical change in a colorant is also dependent upon the spectral distribution of the light source, in particular the proportion of radiation of wavelengths most effective in causing a change in the colorant and the quantum yield of colorant degradation as a function of wavelength. On the basis of photochemical principles, it would be expected that light of higher energy (short wavelengths) would be more effective at causing fading than light of lower energy (long wavelengths). Studies have revealed that this is not always the case. Over 100 colorants of different classes were studied and found that generally the most unstable were faded more efficiently by visible light while those of higher lightfastness were degraded mainly by ultraviolet light (McLaren, K., J. Soc. Dyers Colour, 1956, 72, 86).
The influence of a substrate on colorant stability can be extremely important. Colorant fading may be retarded or promoted by some chemical group within the substrate. Such a group can be a ground-state species or an excited-state species. The porosity of the substrate is also an important factor in colorant stability. A high porosity can promote fading of a colorant by facilitating penetration of moisture and gaseous reactants into the substrate. A substrate may also act as a protective agent by screening the colorant from light of wavelengths capable of causing degradation.
The purity of the substrate is also an important consideration whenever the photochemistry of dyed technical polymers is considered. For example, technical-grade cotton, viscose rayon, polyethylene, polypropylene, and polyisoprene are known to contain carbonyl group impurities. These impurities absorb light of wavelengths greater than 300 nm, which are present in sunlight, and so, excitation of these impurities may lead to reactive species capable of causing colorant fading (van Beek, H.C.A., Col. Res. Appl., 1983, 8(3), 176).
Therefore, there exists a need for methods and compositions which are capable of stabilizing a wide variety of colorants from the effects of both sunlight and artificial light.